A monthslong calm on Wall Street shattered on Friday, and U.S. stocks are falling after President Donald Trump threatened to crank tariffs much higher on China.

The S&P 500 sank 2.1% and was tumbling toward its worst day since April. The Dow Jones Industrial Average was down 668 points, or 1.4%, with an hour remaining in trading, and the Nasdaq composite was 2.8% lower.

Stocks had been heading for a slight gain in the morning, until Trump took to his social media platform and said he’s considering “a massive increase of tariffs” on Chinese imports. He’s upset at restrictions China has placed on exports of its rare earths, which are materials that are critical for the manufacturing of everything from consumer electronics to jet engines.

“We have been contacted by other Countries who are extremely angry at this great Trade hostility, which came out of nowhere,” Trump wrote on Truth Social. He also said “now there seems to be no reason” to meet with China’s leader, Xi Jinping, after earlier agreeing to do so as part of an upcoming trip to South Korea.

The ratchet higher in tensions between the world’s two largest economies led to widespread drops across Wall Street, with four out of every five stocks within the S&P 500 falling. Everything sank from Big Tech companies like Nvidia and Apple to stocks of smaller companies looking to get past uncertainty about tariffs and trade.

The market may have been primed for a slide. U.S. stocks were already facing criticism that their prices had shot too high following a nearly relentless 35% run for the S&P 500 from a low in April to record heights.

Critics say the market looks too expensive after prices rose much faster than corporate profits. Worries are particularly high about companies in the artificial-intelligence industry, where pessimists are making comparisons to to the 2000 dot-com bubble that ultimately imploded. For stocks to look less expensive, either their prices need to fall, or profits need to rise.